The present invention relates to a thermal head for a thermal printer, and more particularly to a thermal head which is well suited for raising a printing speed and enhancing a printing quality.
In general, a thermal head is such that, as described in U.S. Pat. No. 4,517,444, Electronics/Aug. 5, 1976, etc., a substrate made of ceramics or the like is provided with a heat accumulating layer, on the surface of which a plurality of minute heating resistors are arranged.
When the printing cycle is shortened in order to raise the printing speed of the thermal printer of this type, the next printing operation starts before the head cools completely, and the temperature rises gradually each time the printing operation is repeated. Consequently, as the printing is repeated, the printing density rises gradually. Another disadvantage is the occurrence of, e. g., the so-called trailing phenomenon in which, even after the printing has been ended, it continues for a while because the temperature of the head does not lower. Such phenomena become conspicuous when the printing cycle becomes shorter than about 5 ms, and they are very conspicuous for a printing cycle shorter than 1 ms. The reason is that the period of time to be assigned to cooling shortens as the printing cycle becomes shorter.
Such phenomena, which are very unfavorable for the thermal printer, are ascribable to the inferior thermal response characteristic of the thermal head, the cause of which is considered to lie in the heat accumulating layer. The heat accumulating layer is a kind of heat insulating layer which is disposed lest heat generated by the heating portion of the heating resistor should be radiated through the substrate of good heat conduction. Accordingly, it is greatly effective to employ a material with which the temperature conductivity k (m.sup.2 /s) of the heat accumulating layer becomes nearly equal to, or desirably, lower than that of a protective layer situated on the opposite side of the heat accumulating layer with the heating resistor intervening therebetween. In general, SiO.sub.2, Ta.sub.2 O.sub.5 etc. are employed as the materials of the protective layer, and the temperature conductivities thereof are on the order of 1.times.10.sup.-6 m.sup.2 /s. Accordingly, the heat accumulating layer is usually made of a material difficult of conducting heat, the temperature conductivity k (m.sup.2 /s) of which is not higher than 1.times.10.sup.-6 m.sup.2 /s. It is considered that, in the prior-art thermal head, the heat accumulating layer will be unnecessarily thick and will therefore act as a thermal resistance during the cooling, to induce the disadvantages mentioned before. Heretofore, the thermal characteristic of a thermal head has not been considered in regard to the thickness of the heat accumulating layer of the head.